Tunable magnetrons



March 27, 1962 D. L, WINSOR TUNABLE MAGNETRONS 2 Sheets-Sheet 1 FiledNov. 5, 1958 M R mw WTW w/ 0 w m D A TTOR/VEY March 27, 1962 Filed NOV.3, 1958 D. L. WINSOR TUNABLE MAGNETRONS 2 Sheets-Sheet 2 lNVENTOR DONALDL. Wl/VSOR WWM Uite

This invention relates to electron discharge devices of the magnetrontype and more particularly pertains to a magnetron incorporating afrequency tuning mechanism.

Magnetrons are particularly useful for generating high frequency energyat extremely short wave length. A particular form of magnetron overwhich this invention is an improvement comprises an anode blockconstituted of copper or some other highly electrically conductivenon-magnetic material shaped in the form of a right circular cylinderhaving planar ends perpendicular to the axis of the cylinder. A centralaxial cylindrical aperture is provided in the anode block which housesan axially disposed electron emissive cathode. A plurality of resonatorcavities are provided in the anode block, the resonator cavitiesextending parallel to the axis of the cylinder and being arranged abouta circle whose center lies on the axis of the cylinder. Each of theresonator cavities communicates with the central aperture through aradially extending slot. This construction is commonly known as amulti-cavity magnetron.

It is well known that conventional multi-cavity magnetrons in which thecavity resonators are all alike oscillate in any one of a number ofdifferent modes, each of the different modes of oscillation involving aninstantaneous phase difference between the oscillations generated inadjacent cavities. It has been found that the most efficient anddesirable mode of operation is that referred to as the Gr-mode in whichthe oscillations in any arbitrarily selected cavity of the magnetron is180 (1r radians) out of phase compared with the phase of theoscillations in the adjacent cavity. Another way of visualizing vr-modeoperation is to view the cavity resonators as comprising two sets, oneset of cavity resonators consisting of any arbitrarily selected cavityand every alternate cavity, the other set consisting of the remainingcavities, and to consider all the cavities in each set to oscillate inphase with all the other cavities in that set so that when theoscillations in the cavities of one set are at maximum positivepotential, the oscillations in the cavities of the other set are atmaximum negative potential. In order to achieve 1r-rnode operation incon ventional multi-cavity magnetrons it is customary to electricallyconnect alternate anode elements by means of low impedance conductors orstraps which effectively lock alternate cavities together in the samephase. One effect of these straps is to shift the oscillationfrequencies of the various undesirable modes away from the vr-rnodewithout affecting the latter, thus facilitating the elimination of theundesirable modes by causing the electric and magnetic field conditionsfavoring the generation of the undesirable modes to be further removedfrom the conditions which favor vr-mode operation.

It is also well known that the frequency of oscillation of aconventional magnetron is determined by the geometry of the cavityresonators and that the magnetron may be tuned in frequency by insertingdevices in the cavities which alter their capacitance or inductance. Asthe trend in the electronic art has demanded generators capable ofoperating at higher and higher frequencies, the magnetron, in an effortto meet that demand, has been designed to generate such higherfrequencies by scaling down the geometry of the cavity resonators. As aresult of the diminution in the size of the cavity resonators closertolerances are required, and the devices which are States Patent 9 "iceinserted into the cavities to eifect tuning of the magnetron have becomeless massive and less rigid in a situation where more rigidity isnecessary due to the very close spacing required. In addition, thetuning devices, because of their decreased mass, are less able todissipate the heat generated within the magnetron so that deformation ofthe devices is likely to occur due to their overheating.

This invention discloses a magnetron construction permitting the largestpossible cavity geometry for a desired resonant frequency and proposesaffixing covers to the ends of the anode block to enclose the resonantcavities, at least one of those end covers having aperturescircumferentially coincident with the profile of a portion of eachcavity. In accordance with the invention, tuning pins, secured at oneend to a movable plate, are arranged to project through the apertures inthe end cover and penetrate into the cavities. Apparatus is provided forraising or lowering the movable plate, in order that the tuning pins maybe advanced into or withdrawn from the cavities to cause a change in theresonant frequency of those cavities. By virtue of this novelconstruction, it has been found that increased effectiveness in changingthe frequency of oscillation is obtained for a given penetration of thetuning pins. As a corollary to the increased effectiveness of the tuningpins, greater tuning ranges are achieved without any significantincrease in the complexity of the tuning mechanism.

The invention further proposes that one or both of the anode end coversbe provided with slots disposed to permit the propagation of energy ofthe adjacent mode (that is, the mode next adjacent the desired mode ofoscillation) into the magnetron end space. By properly orienting suchslots, it is possible to couple the adjacent mode to the magnetron endspace to load it sufiiciently to suppress oscillation in that mode.

The invention, together with its manner of construction and mode ofoperation, will be better understood by a perusal of the followingdescription when considered in conjunction with the drawing wherein:

FIG. 1 is a vertical section through a magnetron constructed inaccordance with the invention;

FIG. 1a is a horizontal section taken through said magnetron withcathode and tuning pins removed;

FIG. 2 is a cross-section through an anode block showing theconfiguration of the resonant cavities;

FIG. 3 depicts a segment of an anode having end covers; and

FIG. 4 depicts a modification of the structure shown in FiG. 3.

Referring now to PEG. 1 which depicts a longitudinal section through amagnetron constructed in accordance with the invention, there is shownan anode block 1 of copper or some other highly electrically conductivematerial enclosed within an evacuated chamber formed in part by atubular body 2. As may be seen in FIG. 2 the anode has resonant cavities3 therein which communicate with a central chamber 4, all the cavityresonators being alike in configuration. The cavities are teardrop incross section, the rear wall 5 of each cavity being circular and thewalls formed by the vanes 6 being tangent to that circular wall portion.FIG. 3 shows the anode block 1 in a position inverted to that depictedin FIG. 1. One set of alternate vanes 6 of the anode are interconnectedby straps 7 and straps 8 interconnect the other set of alternate vanes.Those straps are provided for the purpose of achieving better modeseparation in the magnetron in order to prevent oscillations in anundesired mode from occurring. In effect, the straps tend to insure1rmode operation of the magnetron by causing one set of alternateresonant cavities to oscillate in a phase which is 7r radians displacedfrom the phase of oscillations in the other set of resonant cavities.Secured to one end of the anode block 1 is an annular cover or plate 3,illustrated in both FIGS. 1 and 3'. The plate 9 at its oute periphery isbrazed or otherwise secured to the anode l and as best seen in FIG. 1,that plate does not rest upon the vanes 6 or close off the upper ends ofthe cavity resonators but is separated therefrom by a small space. Thisplate 9 is used to minimize the effects of tuner resonance. The otherend of the anode has secured to it a cover or plate ill which abuts thevanes and closes the resonant cavities at that end. Each of the covers 9and It) has circular apertures ll therein, the number of such aperturesin each cover corresponding to the number of cavity resonators in theanode. The diameter of the apertures 11 is identical with the diameterof the circular wall portion of the resonant cavities and each apertureis arranged in relation to the adjacent resonant cavity so that aportion of the circular edge of the aperture is coincident with theperiphery of the circular wall portion of the resonant cavity. Thuscorrespondingly positioned apertures in the covers 9 and 1:) arealigned. Tuning pins 12, shown in PEG. 1, are supported from a carrier13 and each pin is arranged to be inserted through an aperture in cover9 into a resonant cavity. The tuning pins are circular in cross sectionand are of smaller diameter than the diameter of the apertures. it isimportant that each tuning pin be centrally disposed within itsapertures and not in contact with either the cover 9 or the wall of theresonant cavity. To obtain the requisite alignment of the tuning pinsduring assembly of the magnetron an alignment jig may be insertedthrough the apertures in the oppositely disposed cover to position thepins in the desired positions. Upon completion of the necessarymanufacturing operations the alignment jig is removed. The effect of thecover in during operation of the tube is to cause the magnetic componentof tr e fields the resonant cavities to be displaced in the direction ofthe cover 9 so that the penetration of the tuning pins into the resonantcavities has an enhanced effect in altering the inductance of theresonant cavities and hence an increased effectiveness in tuning thefrequcncy of the magnetron. That is, penetration of a tuning pin into aresonant cavity effects a volume displacement of the magnet fieldcomponent in that cavity which changes the inductance of the resonantcavity and due to the presence of cover It the magnetic field componentis more intense at the tuner end of the anode so that a givenpenetration of the tuning pin has a larger influence than it would havein the absence of the cover 19.

The employment of end covers 9 and 1% causes a corn siderable increasein the resonant frequency of the cavity resonators 3 so that for a givenresonant frequency a larger cavity geometry is made possible. That is,in the absence of end covers the geometry of the cavity esonators mustbe of a certain size to cause resonance at a desired frequenc theaddition of end covers, however. permits the same resonant frequency tobe obtained with cavity resonators of the same configuration but oflarger size. This effect of the end covers is significant where themagnetron is designed to operate at very high frequencies, for example,frequencies in the vicinity of 10,080 megacycles (X-band), because atsuch frequencies the size of the cavity resonators becomes very smalland difliculties are encountered in adhering to mechanical tolerances.For manufacturing reasons, therefore, it is highly desirable that thegeometry of the cavity resonators be as large as possible. Theattainment of larger cavity geometry permits, as a corollary advantage,the use of tuning pins having the largest possible cross-sectional area.These larger tuning pins are more rigid than pins of smaller crosssectional area and are better able to dissipate the heat generated inthe interior of the magnetron. The end covers also function to limit thefringing RF (radio requency) fields at the ends of the anode block andthereby reduce the sensitivity of the magnetron to the geometry of theend spaces, viz., the spaces in the enclosed chamber at ends of theanodes. End covers also permit the use of long anode structures withouttrouble from inadequate separation between mode multiplcts, modemultiplets being the families of modes having a given order of RFvoltage variation along the length of the anode. As is understood bythose familiar with mag netrons, the length of the anode is the distancebetween the planar ends of the anode block.

in order to suppress any tendency of the magnetron to oscillate in themode (N being the number of cavity resonators) which is the mode nextadjacent the 1r mode, radial slots 5.4 may be added in the end cover it)and so oriented as to couple the adjacent mode into the end space whereit is loaded sufiiciently to prevent oscillation in that mode from building up to any significant extent. The best orientation of the slots forthe suppression of the adjacent mode is attained by arranging the slotsalong radial lines which intersect at a 45 angle a diametral linepassing through the cavity in the anode block from which the output ofthe magnetron is taken as shown in FIG. 1a.

Referring again to FIG. 1, there is shown a mechanism for raising andlowering the carrier 13 and its attached tuning pins. The carrier 13 issecured to a piston 15 at one end of a rod .16, the rod being providedwith micrometer screw threads 17 at its other end which are engaged by athreaded plate 18 attached to a circular plate or wheel 19 provided atits periphery with means such as ltnurling so that it may be readilymoved. The periphery of threaded plate 18 constitutes a ball-bearingrace. Surrounding that race is a complementary race comprising twobevelled rigs 2t and 21, the ring 2% being seated upon a stationary disc22, and the ring 21 being pressed into cooperation with ring 2-0 by anannular spring 23 affixed to disc 22. Ball bearings are positionedbetween the races to permit the wheel 19 to rotate easily. The disc 22is secured to a flange 24 fastened to pole piece 25, this latter memberfunctioning as a cylinder for piston 15. To insure a lealoproofconnection between piston and cylinder, a Sylphon bellows 26 is providedhaving an outwardly extending rim at one end which is soldered to aninternal shoulder of pole piece 25, the opposite end of the bellows 25being bonded to the piston 15. The mechanism described above for raisingand lowering the tuning pins is exemplary only and it is not intendedthat the invention be limited to the illustrated mechanism since,obviously, many other mechanisms are available for performing the samefunction.

Disposed in the central chamber of the anode block is a cathodestructure 2'7 which includes a cathode sleeve preferably of nickel,having a diametrically reduced portion substantially coextensive withthe length of the anode block, the surface of the reduced portion beingcoated with an electron-ernissive substance, for example, an alkalineearth metal oxide, which. copiously emits electrons at elevatedtemperatures. Housed within the cathode sleeve 23 in a location bestsuited for the transfer of heat to the electron-emissive coating is aheater filament 29', one end of the filament being in electrical contactwith the sleeve 28. The upper end of the cathode sleeve is disposedwithin the end space of the magnetron housing and piston 15 isconstructed as a hollow shell so that when the piston is lowered thesleeve 2% is received within the hollow without contacting the walls ofthe piston. The lower end of cathode sleeve 23 is centrally disposedwithin another pole piece 39. The cathode sleeve, of course, isinsulated from the pole piece 3-). A magnetic field is establishedbetween the pole pieces 25 and 3% by connecting the pole pieces to theends of a horseshoe magnet 31, the end portions only being illustrated.The magnetic field extends through the interaction region of themagnetron which is the space between the activated surface of thecathode and the inner ends of the vanes 6.

The output of the magnetron may be obtained in a conventional manner, asby inserting an antenna loop through an aperture in the wall of themagnetron into one of the cavity resonators and omitting the tuning pinnormally positioned in that cavity. Where the magnetron is designed togenerate frequencies in the shorter wavelength regions, however, such asat X-band, an antenna loop becomes inpracticable and a waveguide outputis commonly employed. FIG. 1 illustrates a magnetron having a waveguideoutput. A waveguide section 32 is secured to the housing of themagnetron and an aperture 32:: is provided in the anode block 1 whichpermits electromagnetic energy in one of the cavity resonators topropagate into the waveguide. In order that the vacuum in the magnetronmay be maintained, the outer end of the waveguide section 32 is sealedby a dielectric window 33.

in order to remove the heat generated by the magnetron, the tubular body2 is provided with a passage 34 through which water or some othercoolant may be circulated. Suitable inlet and outlet connections (notshown) must be provided through which the coolant may be introduced intoand withdrawn from the passage. This means for cooling a magnetron isconventional and some other means of cooling the magnetron may well beemployed, if desired.

FIG. 4 illustrates a modification of the invention and shows an anodeblock 40 in which end covers 41 and 42 are secured to the block andenclose the ends of the resonant cavities. For purposes of comparison,it will be noted that in the structure illustrated in FIG. 3 the endcover 9 does not abut the edge of the cavity vanes 6 but rather isspaced from the edge of the cavity vanes, whereas in the structureillustrated in FIG. 4, each end cover, 41 and 42, is bonded to and abutsthe edge of vanes 43 along a larger extent of the edge of the vanes. Inall other essential respects the structures shown in FIGS. 3 and 4 areidentical. End covers 41 and 42 are provided with apertures in the samemanner that end covers 9 and 10 are provided with apertures. Thestructure illustrated in PEG. 4 is advantageous in that by enclosingboth ends of the resonant cavities the resonant frequency of thecavities is increased to a greater extent than is possible where onlyone end is enclosed, thereby permitting the largest attainable cavitygeometry for a given resonant frequency. However, the symmetricalconstruction shown in FIG. 4 causes the magnetic field components in thecavity resonators to have a symmetrical distribution so that themagnetic field components are not shifted toward the tuner end of theanode, as they are in the structure of FIG. 3. Therefore, for a givendepth of tuner penetration into a cavity resonator, the amount offrequency shift which is obtained with the construction of FIG. 4 is notas great as the amount of frequency shift obtained with the constructionof FIG. 3.

While there have been illustrated only two embodiments of the invention,it will be apparent that the invention is not limited to the precisestructures depicted in the drawings and that variations are apparent tothose skilled in the magnetron tube art. For example, the end cover 10shown in FIG. 3 and the end cover 42 in FIG. 4, which is opposite thetuner end, need not have alignment apertures therein, but may beimperforate annular discs. As further examples, the tuning pins need notbe circular in cross-section but may be of a different configuration andthe cavity resonators need not be teardrop in shape. Therefore, it isintended that the scope of the invention be construed in accordance withthe appended claims.

What is claimed is:

1. A frequency tunable electron discharge device of the magnetron typecomprising a cylindrical anode block having a central chamber extendingalong the longitudinal axis thereof, said anode block having a pluralityof anode segments partially defining resonant cavities thereincommunicating with said central chamber, a cathode situated in saidcentral chamber, at least one end cover contacting an end face of saidanode segments, each end of said anode block having a cover attachedthereto, at least one of said covers having apertures therein permittingaccess to said resonant cavities, a portion of the edge configuration ofeach aperture being coincident with a portion of the profile of theadjacent resonant cavity, a plurality of tuning pins disposed at one endof said anode block, each of said tuning pins extending through adifferent one of the apertures in the adjacent end cover into one ofsaid cavities, and means secured to said tuning pins for adjusting thedepth of penetration of said pins in said resonant cavities.

2. A frequency tunable electron discharge device of the magnetron typecomprising a cylindrical anode block having a central chamber extendingalong the longitudinal axis thereof, said anode block having a pluralityof anode segments partially defining resonant cavities thereincommunicating with said central chamber, a cathode situated in saidcentral chamber, at least one end cover contacting an end face of saidanode segments, each end of said anode block having a cover attachedthereto, at least one of said covers having apertures therein permittingaccess to said resonant cavities, a portion of the edge configuration ofeach aperture being coincident with a portion of the profile of theadjacent resonant cavity, at least one of said end covers having radialslots therein for coupling undesirable mode energy into an end space, aplurality of tuning pins dispposed at one end of said anode block, eachof said tuning pins extending through a different one of the aperturesin the adjacent end cover into one of said cavities, and means securedto said tuning pins for adjusting the depth of penetration of said pinsin said resonant cavities.

3. A frequency tunable electron discharge device of the magnetron typecomprising a cylindrical anode block having a central chamber extendingalong the longitudinal axis thereof, said anode block having a pluralityof anode segments partially defining resonant cavities thereincommunicating with said central chamber, a cathode situated in saidcentral chamber, at least one end cover contacting an end face of saidanode segments, each end of said anode block having a cover attachedthereto, at least one of said covers having aperture therein permittingaccess to said resonant cavities, a portion of the edge configuration ofeach aperture being coincident with a portion of the profile of theadjacent resonant cavity, a plurality of tuning pins disposed at one endof said anode block, each of said tuning pins extending through adifferent one of the apertures in the adjacent end cover into one ofsaid cavities, each of said tuning pins having a cross-sectionalconfiguration identical with and of smaller size than the aperturethrough which it extends, and means secured to said tuning pins foradjusting the depth of penetration of said pins in said resonantcavities.

4. A frequency tunable magnetron comprising an evacuated housing, acylindrical anode block disposed in said housing, a cathode, said anodeblock having a central chamber in which said cathode is situated, saidanode block having a plurality of anode segments partially definingresonant cavities therein communicating with said central chamber, afirst cover in said housing contacting one end face of said anodesegments, a second cover in said housing secured to the other end faceof said anode segments and enclosing said resonant cavities, said covershaving apertures therein permitting access to said resonant cavities, aportion of the edge configuration of each aperture in said cover beingcoincident with a portion of the profile of the adjacent resonantcavity, a plurality of tuning pins, each of said tuning pins extendingthrough a different one of the apertures in said first cover into one ofsaid cavities, and means secured to said tuning pins for concurrentlyadjusting the depth of penetration of said pins in said resonantcavities.

5. A frequency tunable magnetron comprising an evacuated housing, acylindrical anode block disposed in said housing, a cathode, said anodeblock having a central chamber in which said cathode is situated, saidanode block having a plurality of anode segments partially definingresonant cavities therein communicating with said central chamber, afirst cover in said housing contacting one end face of each of saidanode segments, a second cover in said housing secured to the other endface of said anode segments and enclosing said resonant cavities, saidcovers having apertures therein permitting access to said resonantcavities, a portion of the edge configuration of each aperture in saidcover being coincident with a portion of the profile of the adjacentresonant cavity, said second cover being additionally provided withradial slots for coupling undesirable mode energy to the end space insaid housing, a plurality of tuning pins, each of said tuning pinsextending through a different one of the apertures in said first coverinto one of said cavities, and means secured to said tuning pins forconcurrently adjusting the depth of penetration of said pins in saidresonant cavities.

6. A frequency tunable magnetron comprising an evacuated housing, acylindrical anode disposed in said housing, a cathode, said anode havinga central chamber in which said cathode is disposed, said anode having aplurality of anode segments partially defining resonant cavities thereincommunicating with said central chamber,

a pair of end covers, said covers being secured to opposite end faces ofsaid anode segments and enclosing said resonant cavities, at least oneof said covers having apertures therein providing access to saidresonant cavities, a portion of the edge configuration of each aperturebeing coincident with a portion of the profile of the adjacent resonantcavity, a plurality of tuning pins, each of said tuning pins extendingthrough a different one of said apertures into a resonant cavity, andmeans secured to said tuning pins for concurrently adjusting the depthof penetration of said pins in said resonant cavities.

7. A frequency tunable magnetron comprising an evacuated housing, acylindrical anode disposed in said housing, a cathode, said anode havinga central chamber in which said cathode is disposed, said anode having aplurality of anode segments partially defining resonant cavities thereincommunicating with said central chamber, a pair of end covers, saidcovers being secured to opposite end faces of said anode segments andenclosing said resonant cavities, at least one of said covers havingapertures therein providing access to said resonant cavities, a portionof the edge configuration of each aperture being coincident with aportion of the profile of the adjacent resonant cavity, at least one ofsaid end covers having radial slots therein oriented to coupleundesirable mode energy into an end space in said housing, a plurmity oftuning pins, each of said tuning pins extending through a different oneof said apertures into a resonant cavity, and means secured to saidtuning pins for concurrently adjusting the depth of penetration of saidpins in said resonant cavities.

8. An electron discharge device of the magnetron type comprising acylindrical anode block having a central chamber extending along thelongitudinal axis thereof, said anode block having a plurality of anodesegments partially defining resonant cavities therein communicating withsaid central chamber, a cathode situated in said central chamber, atleast one end cover having one opening concentric with said cylinder anda plurality of smaller openings each aligned with one of said cavitiesand substantially smaller than the end of the cavity contacting an endface of said anode segments and radial slots extending from at least oneof said smaller openings to said one opening.

9. An electron discharge device of the magnetron type comprising acylindrical anode block having a central chamber extending along thelongitudinal axis thereof, said anode block having a plurality of anodesegments partially defining resonant cavities therein communicating withsaid central chamber, a cathode situated in said central chamber, atleast one end cover having one opening concentric with said cylinder anda plurality of smaller openings each aligned with one of said cavitiesand substantially smaller than the end of the cavity contacting an endface of said anode segments, said end cover having a. plurality ofradial slots therein, said slots extending from different of said smallopenings to said one opening, the width of said slots beingsubstantially smaller than the width of said cavities for couplingundesirable mode energy into an end space.

10. An electron discharge device of the magnetron type comprising acylindrical anode block having a central chamber extending along thelongitudinal axis thereof, said anode block having resonant cavitiestherein cornmunicating with said central chamber, a cathode situated insaid central chamber, at least one end cover having one openingconcentric with said cylinder and a plurality of smaller openings eachaligned with one of said cavities and substantially smaller than the.end of the cavity attached to an end of said anode block, said end coverhaving a plurality of radial slots therein, said slots extending fromdifferent of said small openings to said one opening, the width of saidslots being substantially smaller than the width of said cavities forcoupling undesirable mode energy into an end space.

References Cited in the file of this patent UNITED STATES PATENTS2,462,510 Korman Feb. 22, 1949 2,508,576 Kusch May 23, 1950 2,657,334West Oct. 27, 1953 2,738,441 Dorney et a1. Mar. 13, 1956 2,797,361 GlassJune 25, 1957 2,824,999 Walker Feb. 25, 1958 FOREIGN PATENTS 554,552Canada Mar. 18, 1958

